Contamination is a continuing problem in the manufacture of semiconductor devices. Demands for higher product performance have resulted in device designs with narrower line-widths and thinner dielectric materials for faster operation and lower operating voltages. Consequently, the devices are more sensitive to defects and contaminants. Semiconductor manufacturers, therefore, must achieve tighter controls on contamination to meet the electrical performance levels that future generations of products will demand.
Contaminants can be any form of matter that causes unintentional changes in electrical properties of semiconductor devices. Some common contaminants include atomic, ionic, and molecular defects. These contaminants may affect devices by changing leakage, surface inversion, parametric shifts, or reliability characteristics. Electrically active mobile ion contaminants in semiconductor films, in particular, must be minimized since the embedded ion contaminants may attract or repel free charges in the underlying substrate. In most cases, device performance depends on a concentration of free charges in the substrate. The presence of a high level of ion contaminants in the semiconductor films, therefore, will usually introduce unwanted variations in device performance. Sodium ions, for instance, are particularly problematic. Sodium ions are practically invisible and may move unpredictably through the semiconductor film, causing the device to perform improperly or to fail prematurely.
To prevent contaminants from being introduced into the semiconductor films, a semiconductor manufacturer may monitor the level of contaminants in the manufacturing equipment using electrical tests (e.g., capacitance-voltage testing or triangular voltage sweep testing) or analytical tests (e.g., secondary ion mass spectrometry or vapor phase decomposition-inductively coupled plasma mass spectrometry) prior to device processing. Equipment exhibiting an unacceptably high level of contaminants must then be carefully cleaned. Some equipment, plasma etches, for instance, are highly susceptible to contamination. Plasma etching subjects a wafer in a chamber to a chemical plasma and uses a glow discharge to produce chemically reactive substances from a relatively inert molecular gas. The substances then react chemically to etch unprotected areas of the wafer. Etching, by its very nature, removes material from the wafer. Some of the material removed will inevitably deposit in the chamber to contaminate subsequent wafers. The material removed may also deposit on and thereby contaminate the wafer being etched. Careful cleaning of the chamber, therefore, may reduce the level of contaminants in subsequent wafers but may not reduce the level of contaminants in the wafer being etched.
Alternatively, a sensitivity of the device to contamination may be reduced through the use of gettering schemes, such as HCl introduction during thermal oxidation growth, or phosphorous backside gettering. Backside gettering methods such as phosphorous diffusion may trap some contaminants on the backside of the wafer. A disadvantage of phosphorous diffusion, however, is that large amounts of phosphorous on the wafer backside can cause problems in later process steps by contaminating or auto-doping epitaxial layers on the active side of the wafer.
Of course, electrical tests such as capacitance-voltage testing and triangular voltage sweep testing, may be also used after processing to screen out contaminated devices. While reliability may be improved by simply screening out devices having a high level of contaminants, this approach does not actually decrease the level of contaminants in the devices.
Accordingly, what is needed in the art is a method for reducing a level of contaminants from the semiconductor film, thereby increasing product performance and reliability.